Major depressive disorder (MDD) affects millions of people worldwide. Most drugs available to treat MDD target brain pathways involving monoaminergic neurotransmitters. Because up to fifty percent of patients do not improve in response to existing therapies, new treatments that target novel mechanisms are needed. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels mediate Ih, an important current for controlling neuronal excitability. Brain HCN channels are tightly regulated by an auxiliary subunit, TRIP8b (tetratricopeptide repeat-containing Rab8b-interacting protein). Animals lacking TRIP8b and other HCN subunits exhibit an antidepressant-like phenotype, suggesting that inhibiting HCN channels could effectively treat depression. Because the channels are critical in controlling heart rate, directly targeting HCN channels throughout the body is not a viable therapeutic approach. We recently demonstrated that viral overexpression of TRIP8b in the hippocampus leads to changes in HCN channel function with concomitant changes in behaviors associated with antidepressant use. In particular, restoring TRIP8b expression in the hippocampi of TRIP8b knockout mice promoted depression- like behaviors, while a version of TRIP8b with impaired binding to HCN channel pore-forming subunits increased antidepressant-like behavior. These results indicate that the TRIP8b-HCN interaction might be a novel pharmacological target for treating MDD. Although our recent studies with viral gene therapy strengthen the evidence that inhibiting brain HCN might be useful for depression treatment, the approach requires surgery and the technology is not readily translatable to human patients with MDD. On the other hand, inhibitors of the TRIP8b-HCN interaction offer increased ease of use in probing the role of HCN channels in behavior as well as greater translatability as potential therapies. Along theses lines, we have synthesized a small (11 amino acids) peptide capable of binding TRIP8b and blocking the TRIP8b-HCN interaction in vitro. A cell permeable version of this peptide (SNL-CP) was synthesized to allow the peptide to penetrate the blood-brain barrier and cell membranes. In preliminary studies, we show that SNL-CP crosses the plasma membrane of CA1 neurons. We hypothesize that peptide mediated inhibition of TRIP8b's interaction with HCN pore-forming subunits will promote antidepressant-like behavior. In aim 1, we will assess the ability of SNL-CP to disrupt the TRIP8b-HCN interaction after chronic intrahippocampal delivery in mice. These experiments will determine the dosing strategy necessary for therapeutic intervention. In aim 2, we will evaluate the ability of SNL-CP to promote antidepressant-like behavior in mice. The work described in this proposal will evaluate a novel approach to producing antidepressant-like behaviors in mice and could pave the way for a new approach to treating MDD in human patients refractory to existing therapies.